Bola Idor in Gin with Api Keys

Broken Object Level Authorization (BOLA) occurs when an API exposes object identifiers (IDs) in URLs or parameters and fails to enforce that the requesting user is authorized to access that specific object. In Go APIs built with the Gin framework, this risk is amplified when API keys are used for authentication but authorization checks are incomplete or inconsistent.

Consider a Gin endpoint that retrieves a user profile by ID: /api/v1/users/{id}. If the handler validates the presence of an API key but does not verify that the key’s associated user matches the requested {id}, an unauthenticated or low-privilege actor can iterate through numeric IDs and access other users’ data. This is a classic vertical or horizontal privilege escalation scenario. The API key may identify the client application or a service account, but if the handler does not bind that key to the specific resource owner, the authorization boundary is missing.

Gin’s context pattern makes it easy to extract keys from headers, but developers must explicitly enforce scoping. For example, using a key to identify a service or tenant is common, yet failing to cross-check that key against the resource’s ownership or tenant ID enables BOLA. Attackers can exploit this by changing numeric IDs in GET, PUT, PATCH, or DELETE requests. In JSON:API designs, compound identifiers or opaque keys do not inherently prevent BOLA unless the server maps them to permissions.

Moreover, middleware that sets user claims or permissions from the key must ensure those claims are used in every data-fetching operation. A common mistake is reading the key once to authenticate, then relying on route parameters alone to authorize. Gin handlers should always load the resource with a filter that includes the authenticated subject or tenant derived from the key, not just validate the key’s existence.

Real-world parallels include misconfigured REST APIs where numeric IDs are predictable and API keys are treated as ownership proof. Without per-request authorization checks, attackers can chain a valid key with arbitrary IDs to perform BOLA. This is distinct from broken authentication; the key is valid, but the API incorrectly assumes key scope equals resource scope.

To prevent BOLA in Gin when using API keys, enforce ownership or tenant scoping in every handler and avoid trusting route parameters alone. Below are concrete, idiomatic examples that demonstrate secure patterns.

Example 1: Key-bound user access with explicit ID matching

Store the key’s associated user ID in claims, then ensure the requested resource ID matches.

package main

import (
    "net/http"
    "strconv"

    "github.com/gin-gonic/gin"
)

type Claims struct {
    UserID int `json:"user_id"`
    KeyID  string `json:"key_id"`
}

func GetUser(c *gin.Context) {
    // Assume key validation middleware already set claims
    claims, exists := c.Get("claims")
    if !exists {
        c.JSON(http.StatusUnauthorized, gin.H{"error": "missing claims"})
        return
    }
    reqClaims := claims.(Claims)
    paramID, err := strconv.Atoi(c.Param("id"))
    if err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "invalid user id"})
        return
    }
    if reqClaims.UserID != paramID {
        c.JSON(http.StatusForbidden, gin.H{"error": "access denied to this resource"})
        return
    }
    var user User
    if err := db.Where("id = ?", paramID).First(&user).Error; err != nil {
        c.JSON(http.StatusNotFound, gin.H{"error": "user not found"})
        return
    }
    c.JSON(http.StatusOK, user)
}

This ensures the authenticated subject (from the key) matches the requested ID before querying the database.

Example 2: Tenant-aware scoping with API key metadata

When keys identify services or tenants, scope queries by tenant ID derived from the key.

package main

import (
    "net/http"

    "github.com/gin-gonic/gin"
)

type KeyMetadata struct {
    TenantID string `json:"tenant_id"`
    Scope    string `json:"scope"`
}

func ListRecords(c *gin.Context) {
    keyMeta, ok := c.Get("key_metadata")
    if !ok {
        c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid key"})
        return
    }
    meta := keyMeta.(KeyMetadata)
    var records []Record
    // Always filter by tenant derived from the key, not from user input
    if err := db.Where("tenant_id = ?", meta.TenantID).Find(&records).Error; err != nil {
        c.JSON(http.StatusInternalServerError, gin.H{"error": "unable to fetch records"})
        return
    }
    c.JSON(http.StatusOK, records)
}

By binding the key to a tenant context and filtering at the query layer, you prevent horizontal BOLA across resources belonging to different tenants.

Middleware and key validation tips

• Validate the key early, but do not skip authorization in handlers.
• Store minimal, trustable claims (user ID, tenant ID) from the key to avoid secondary lookup races.
• Use parameterized queries and always include the subject identifier in WHERE clauses.